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Misfire Monitor — I/M Readiness Monitor

Misfire Monitor — Readiness Monitor for Misfires

In short, the I/M Readiness Misfire Monitor is a continuous monitor that is always checking for r engine misfires.

How misfires are detected

The misfire monitor detects misfires in two ways. In vehicles equipped with smart ignition coils, the COP coil itself reports a misfire to the ECM. In other vehicles, the misfire monitor tracks the crankshaft position sensor to detect a slowdown in crankshaft rotation that indicates a cylinder didn’t fire or “contribute” power.

Engine RPM isn’t constant; instead it’s a series of power pulses driven by the power from each piston. When a misfire occurs in a cylinder, that particular cylinder doesn’t contribute its share of power to the engine and the engine RPM slows due to the lack of power. The ECM monitors the time between crankshaft position sensor signals and detects when the slowdown occurs. Since it know exactly when it commanded a spark event to each cylinder, it can determine which cylinder misfired.

Based on the amount of the crankshaft speed slowdown, the ECM can also determine how severe the misfire was.

Misfire types

There are three types of engine misfires, Types 1, 2, and 3.

Type 1 and Type 3 misfires are less severe than Type 2 misfires

Type 1 and Type 3 misfires are two-trip monitor faults. If a misfire is sensed on the first trip, the computer temporarily saves the fault in its memory as a Pending Code. The check engine light isn’t lit at that point. However, if the misfire happens on the second trip, and under similar conditions of engine speed, load and temperature, the computer lights the check engine light and sets the specific cylinder misfire code in its long term memory.

 Type 2 misfires are the most severe type of misfire

A Type 2 misfire happens frequently, and is determined to be severe enough to cause damage to the catalytic converter. When the ECM detects a severe Type 2 misfire, it not only light the check engine light, but causes it to flash once per second as soon as the misfire is detected.  When the misfire is no longer present, the MIL reverts to steady “On” condition.

The Misfire Monitor is supported by both “spark ignition” vehicles and “compression ignition” (diesel) vehicles.

©, 2022 Rick Muscoplat

Posted on by Rick Muscoplat

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